1. Field of the Invention
The present invention generally relates to monopropellant propulsion systems and, more particularly, is concerned with a unique monopropellant plenum propulsion system having fast response thrust capability.
2. Description of the Prior Art
For spacecraft or missile attitude control, small propulsion systems usually of the bipropellant or monopropellant type are provided. Such systems are pulsed for short durations, commonly lasting only a fraction of a second, and many thousands of times during a given mission. A bipropellant type system uses two separate propellants, a fuel and an oxidizer, which are not mixed until they come in contact with each other in a combustion chamber when a thrust pulse is needed. A monopropellant type system contains the oxidizing agent and combustible or decomposable matter in a single substance. One conventional monopropellant is a homogeneous chemical agent, such as hydrazine, which is decomposed in a catalyst bed when the thrust pulse is needed.
Monopropellant type propulsion systems have many advantages over bipropellant type systems, several of which are reduced complexity and fewer steps required to produce thrust which could result in faster response. One monopropellant propulsion system is disclosed in U.S. Pat. No. 4,352,782 to James M. Daly and assigned to Rocket Research Corporation. Basically, the propulsion system of the aforesaid patent includes a catalytic gas generator which has a catalytic bed within a thrust chamber, a means to supply a liquid propellant such as hydrazine to the thrust chamber. The catalyst in the catalytic bed reacts with and converts the propellant into a high temperature gas which exits the catalytic bed and is expanded through a conventional nozzle system to produce thrust. Also previously known are bipropellant generators in which an oxidizer such as N.sub.2 O.sub.4 or F.sub.2 is fed (separately from the propellant) to the thrust chamber to react with the propellant. One such generator is shown in U.S. Pat. No. 3,695,041 issued Oct. 3, 1972 to R. F. Eggers and D. L. Emmons and also assigned to Rocket Research Corporation, and is a two-stage device wherein the oxidizer can selectively either not be provided (in which case the engine is a monopropellant engine) for low thrust or be provided (in which case the engine is a bipropellant engine) for high thrust. The U.S. Pat. Nos. 4,352,782 and 3,695,041 are directed to a special means of confining the bed and a variable thrust engine respectively, but serve as background information to describe the nature of monopropellant gas generators prior to the present invention.
However, even the response time of conventional monopropellant propulsion systems, such as the one briefly described above, is too long for use in certain applications, such as missile defense interceptors. For instance, response time is lengthened by the time lags necessary to: (a) open the propellant valve, (b) fill the valve/injector volume, (c) inject propellant into the catalyst bed, (d) decompose the injected propellant, (e) raise the pressure in the thrust chamber, and (f) accelerate the propellant through the nozzle. Prior attempts to increase response time have concentrated on reducing the individual lags described above by increasing valve speed and/or reducing the injector, catalyst bed, and/or chamber volumes. These approaches are limited in their effectiveness since these volumes are an essential part of the propellant flow path.
Consequently, in order to achieve substantially faster response time, a need exists for a fresh approach to monopropellant propulsion system design so as not to be hamstrung by the requirements of the conventional system which allow only fine-tuning of the system.